Ch. Jung

Atomic structure and composition of the P-rich InP(001) surfaces

Abstract The microscopic structure of the P-rich (2×1)-like surfaces of InP(001) is investigated by soft X-ray photoemission spectroscopy (SXPS) and scanning tunneling microscopy (STM). The samples were grown by metal organic vapor phase epitaxy (MOVPE) and then transferred under ultra high vacuum (UHV) conditions to UHV analysis chambers. STM images show a P-rich as-grown surface after transfer and another less P-rich surface after prolonged annealing at 350°C. The In4d emission line is not affected by the change in surface reconstruction and shows a small surface component shifted by 0.48 eV towards higher binding energies. The line shape of the P2p core level, on the other hand, changes: On the as-grown surface, one surface component which is shifted by 0.98 eV towards higher binding energies is found after transfer, whereas two surface components shifted to higher and lower binding energies, respectively, appear after annealing at 350°C. These results are consistent with the structure models derived from STM for the different P-rich (2×1)-like surfaces.

An explanation for the influence of substituents on the radiationless deactivation of 9,10-substituted anthracenes

Abstract The influence of the substituents on the internal and the heavy-atom-induced external intersystem crossing is investigated for six 9,10-substituted anthracenes in the solvent heptane at 23°C. Fluorescence quantum yields, lifetimes, and rate constants of bimolecular quenching ( k P q ) are determined (by impulse fluorometry and relative intensity measurements) and energy differences Δ E ( S 1 - T n ) are calculated quantum-chemically (apart from dibiphenylylanthracene (DBPA) |Δ E ( S 1 - T 2 )| T 1 and single vibronic states of T 2 , it is shown that a simple logarithmic energy gap law for the intersystem crossing rate constant ( k isc ) in the case of small |Δ E ( S 1 - T 2 )| cannot be expected. The model explains the curve of the ratio k isc / k isc (DBPA) as a function of the calculated Δ E ( S 1 - T 2 ) values, showing a lineshape like a Fano antiresonance. The curve for k q / k 1 (DBPA) exhibits a similar shape.

Note of the INDO geometry optimization of biphenyl

Abstract Recently published results of Rayez and Dannenberg concerning the biphenyl molecule show some inadequacies. Our recalculation gives contrary results, which are in qualitatively good agreement with some CNDO/2 calculations presented recently. Consequently, implictions discussed by Rayez and Dannenberg are not justified and they are in contrast to our experience with CNDO/2 geometry optimizations.

Interpretation of resonance cars and shpolskii spectra with calculated molecular geometries, vibrational frequencies and relative intensities: Chrysene in it's lowest excited singlet and triplet state

Abstract Coherent anti-Stokes Raman scattering (CARS) spectra of excited molecules as well as Shpolskii spectra provide information about geometry changes between ground and excited states. Vibrational frequencies and relative intensities from recently obtained CARS spectra of the chrysene S1 and T1 state and earlier observed Shpolskii spectra are interpreted in terms of molecular geometry and force-field changes by means of quantum-chemical consistent force field (QCFF) and Franck-Condon factor calculations. The comparison of observed and calculated relative intensities indicates a coupling between the S1 and S2 state enhancing some of the vibrational radiative singlet transitions both in absorption and fluorescence spectra whereas within the phosphorescence spectra proportionality to calculated Franck-Condon factors is obeyed. The T1 state is the more loosely bound state and its geometry change is different from that of the S1 state. The resonance CARS transitions in the S1 state are assigned to totally symmetric vibrations getting their intensity by a coupling scheme analogous to the A term of the resonance Raman effect: the relative intensity of a transition is shown to be proportional to the Franck-Condon factor to the higher excited state and to the squared vibrational frequency. Using this relation this state can be identified by means of its finger-print-like intensity pattern.

Interpretation of the vibrational spectra and calculation of the geometries of cycloheptatriene, 7-d-cycloheptatriene and phenyl substituted cycloheptatrienes

Abstract Equilibrium geometries, vibrational frequencies and normal coordinates have been calculated quantum chemically for the strained ring compound cycl oheptatriene and its phenyl substituted derivatives. The calculated geometry of cycloheptatriene agrees with the experimental one. The 1-, 2- and 3-phenyl substituted derivatives have a more nearly planar shape for the seven membered ring than cycloheptatriene itself. Comparison with the experimental vibrational frequencies of cycloheptatriene and 7-d-cycloheptatriene results in a satisfactory interpretation of the vibrational spectra. The three carbon—carbon stretching frequencies of the cycloheptatriene π-system show only minor effects caused by phenyl substitution.

Investigations of the n-GaAs-electrolyte interface using time-resolved photoluminescence

The time-resolved decay of band-to-band photoluminescence for n-GaAs (n 0 =5×10 17 cm -3 and 1×10 18 cm -3 ) in contact with aqueous electrolyte was investigated both experimentally (by a sampling technique) and theoretically (by computer simulation) by varying the excitation intensity with an externally applied potential near the point of zero photocurrent under steady state illumination (-0.5 V (SCE)). The band-to-band luminescence decay was observed experimentally at an excitation wavelength of 636 nm in a range of excitation intensity between 1440 and 0.49 kW cm -2 corresponding to initial excess carrier densities p i between 5.5×10 19 and 2×10 16 cm -3

The vibrational structure of low-temperature absorption and fluorescence spectra of 1,2-dimethylnaphthalene in an n-hexane matrix

Abstract Quasilinear absorption and fluorescence spectra of 1,2-dimethylnaphthalene (1,2-DMN) were obtained in a polycrystalline n-hexane matrix, at 4.2 K. The vibrational analysis of the spectra was carried out and fundamental frequencies of ground and first excited states were determined. The equilibrium geometries, normal coordinates and vibrational frequencies of the 1,2-DMN molecules in the So and S1 states were calculated by applying the QCFF/PI method. The Franck—Condon factors were evaluated and the frequencies and intensities of the observed transitions were compared with the calculated ones. The differences between the vibrational structure of 1,2-DMN and naphthalene spectra below 1100 cm−1 are mainly due to the mixing between ag or b3g naphthalene modes with other skeletal ones having out-of-plane contributions in the low-frequency range. The role of the coupled in-phase torsion of the two ue5f8CH3 groups is discussed.

The electronic structure of the GaAs(100) : H surface: a selfconsistent local-density functional LCAO-investigation

Abstract The electronic structures of free and hydrogen covered As-terminated GaAs(100) surfaces were calculated using a selfconsistent local-density functional LCAO-method with a minimal basis set of optimized atomic orbitals. Comparison with experimental and other theoretical results for the bulk electronic structure prove the realiability of this non-empirical method for electronic states in the valence band region. Surface calculations using the slab model with 1 × 1 geometry give hydrogen-induced and GaAs surface states within those energy regions where they have been found with angle-resolved photoemission.

Quantumchemical Perturbation Expansion for the Estimation of the Reaction Path in Radical-radical Reactions

A perturbation method is presented, suitable for the calculation of the interaction energy between two radicals. It works with restricted Hartree-Fock molecular Orbitals of the isolated radicals in their doublet ground state. The interaction energy comprises a coulombic, an exchange, and a charge-transfer contribution. Within the exchange energy contributions of the doubly and of the singly occupied molecular orbitals can be distinguished. The basic equations are simplified further by semiempirical, especially by PPP-, approximations and applied to the interaction of two benzyl radicals. The favoured dimerization in the ^-position is well reproduced by the calculations.

Interactions of open-shell systems, RHF molecular potential equations

Abstract For a supermolecule with up to two open shells the RHF equation is transformed into coupled equations for the components. This is achieved by introducing a constant number of electrons in the components as additional variation constraint. The interaction energy obtained from these equations represents polarization and exchange-polarization effects besides the first-order energy. As example MINDO/2 calculations are discussed for the systems methyl radical—ethylene, methyl radical—methyl radical, ethylene—ethylene cation.